Operational amplifiers have been widely used in the electronics industry for their many excellent circuit characteristics including high open loop gain, high input impedance, and low output impedance. General applications of the operational amplifier include circuit configurations such as voltage and current amplifiers, differentiators and integrators, comparators, active filters, oscillators, and analog to digital and digital to analog converters. Operational amplifiers may be further used to convert a voltage to a current (transconductance amplifiers) by inputting a voltage to be converted into the noninverting input of the operational amplifier, and having the output drive a Darlington transistor pair which supplies the desired output current. One drawback of the operational amplifier is a necessary trade-off between speed and accuracy. Higher speed is obtained by reducing the DC precision; likewise, a low DC error is achieved at the expense of a reduced bandwidth. Further, the operational amplifier requires differential inputs, compensation, offset correction, external feedback, and a significant amount of silicon area in its implementation.
A more recent circuit development, the operational transconductance amplifier (OTA), has emerged which improves upon some of the operational amplifier characteristics. The OTA, like the operational amplifier, can be used in a wide variety of circuit and system applications. The OTA, however, offers higher performance and its performance is independent of load conditions. A major advantage of the OTA lies in the ability to couple the outputs of several OTAs together in order to sum, subtract, or multiply the voltages at the inputs of the individual OTAs. Because the outputs of the OTAs are currents, coupling additional outputs do not degrade performance. Also, because inputs are isolated, increasing the number of inputs does not degrade performance Like the operational amplifier, however, OTAs require differential inputs and external feedback, a significant amount of silicon area, and are not able to operate linearly with an input voltage which goes all the way to the V.sub.SS or V.sub.DD supply rail. It is desirable to negate the need for external feedback and reduce the number of terminals required by using a single ended input versus the differential inputs required by the operational amplifier and the OTA.
Thus, what is needed is a voltage to current converter providing a reference current, the magnitude of the reference current varying linearly with an input voltage that may vary to the supply voltage rails, having a single ended input stable over a wide bandwidth without requiring compensation, offset correction, external feedback, or capacitive coupling of the input voltages.